This invention relates to mass flowmeters of the type in which the mass rate of flow of the fluid is measured by sensing the temperature differential in the flowing fluid caused by the addition of heat at a generally constant rate. Typical mass flowmeters of this type are disclosed in U.S. Pat. No. 2,729,976 to Laub and U.S. Pat. No. 3,938,384 to Blair. One conventional version of this type of flowmeter utilizes a sensor tube of substantially smaller diameter than the main flow path which is connected to the main flow path at an upstream and a downstream location. Wrapped around the sensor tube is a pair of temperature-sensitive resistance coils which are part of a bridge circuit. A DC power supply applies current to the resistance coils and the heat generated thereby is transferred to the fluid passing through the sensor tube. As the fluid flows from its upstream to its downstream locations within the sensor tube, the electronic circuit, which was initially balanced at zero fluid flow, becomes unbalanced. The unbalance of the electronic circuit is directly proportional to the mass flow rate of the fluid passing through the sensor tube and thus to the mass flow rate through the primary flow path.
Conventional flowmeters of this type suffer from several deficiencies. Because of the typically small internal diameter of the sensor tube, it is subject to contamination by particulate matter in the gas stream. Such extraneous deposits can cause serious measurement errors. Some flowmeters of this type have their tubes integrally connected to the primary flow passage so that the flowmeter must be removed from the flow line in order to clean the sensor tube. Most sensor tubes have a "U", rather than a straight, configuration. Some of these are removable from the primary flow passage by first removing the electronics enclosure, but, because of their "U" geometry, they are very difficult to thoroughly clean either by insertion of a wire or by purging. In addition, because either two separate resistance coils or a single center-tapped sensor coil is utilized, the coils are influenced by heat convection from one another. If the attitude of the flowmeter is other than horizontal, the higher coil will be influenced by upward external heat convection from the lower coil, thereby giving an erroneous reading of flow rate. The Blair patent has attempted to eliminate this deficiency by surrounding the coils with insulating material of open cell foam. However, because the foam material also absorbs heat from the resistance coils, and thus away from the fluid flowing within the sensor tube, the response time of the bridge circuit to changes in fluid flow rate is increased and conductive heat losses occur.